There are many different technologies currently used to build physical three-dimensional models using additive manufacturing technology, e.g. building a model layer by layer. Typically, a virtual design of the three-dimensional model, e.g. represented through computer-aided design (CAD) software or the like, is transformed into a plurality of thin (quasi-two-dimensional) cross-sectional layers which are built on one another in succession.
There are a number of known ways in which the cross-sectional layers may be formed. For example, it is known to sinter a selected sub-region of a deposited area of green material, e.g. by guiding a laser beam over the sub-region. Such an arrangement is disclosed in WO 2004/056512. Another example involves the use of a two-dimensional heat or light source arranged to melt or cure all of a selected sub-region at the same time by masking out the areas not to be melted. Such an arrangement is disclosed in US 2002/0149137.
Other techniques may involve extruding or otherwise depositing green material already in the correct shape of the cross-sectional layer (e.g. by reference to an x-y-z-table constructed from the virtual design data). The deposited material may then harden naturally or be cured (e.g. by a powerful light source) to form the desired cross-sectional layer.
In a yet further example, each desired cross-sectional layer may be cut out of a sheet material, wherein the model is built by gluing together the cut-out layers.
US 2005/208168 discloses a technique in which a desired cross section is formed on the surface of a heated drum and subsequently transferred from the drum onto and fused together with previously deposited layers.
WO 2011/045291 discloses the use of a thermal head as a means of selectively heat-treating successive layers of green material to create a plurality of cross-sectional layers forming a three-dimensional model.
During heat-treatment in this arrangement, there is relative movement between the deposited layer and the heating elements on the thermal head. To prevent the heat-treated material from adhering to the heating elements, which would disrupt the building process and eventually cause overheating and failure of the thermal head, WO 2011/045291 discloses the use of a protective sheet beneath the thermal head, i.e. disposed between the heating elements and the deposited layer.
WO 2011/045291 also discloses the provision of independently heatable covers (e.g. plates) in thermal communication with the surface of the material bed on either side of the print mechanism to control the temperature of the heat-treated material in a manner to prevent warping of the model.